1. Field of the Invention
The present invention is used to eliminate the parasitic effect of the circuit board with mounting pads by etching the area on the ground plane corresponding to the surface-mounted devices on a routing layer for increasing the response of routing traces and raising the quality of the circuit board.
2. Description of Related Art
Since the advancement of IC technique, the required wiring pins are getting more complicated and the more routing traces are needed oppositely. Therefore, the design of a PCB is getting more complicated and integrated due to the area spared and multi-layer design of the PCB.
The method for manufacturing the conventional PCB is to press a plurality of layers with different functions, including at least a power plane, an element layer, a signal layer, a ground plane and some interposed insulator layers. Since the distance of separation ground plane and the routing layer is narrow, the parasitic effect of mounting pads is brought out and exceeds the original design of the PCB.
FIG. 1 shows a schematic drawing of a multi-layer PCB with a routing layer 11 and ground plane 13 in the prior art, wherein the conducting line 12 and the mounting pad 14 connected with are formed on the routing layer 11. The mounting pad 14 is used to solder the surface-mounted device (SMD) 16 above the routing layer 11. The mounting pad 14 can be the place of a lead terminal or connector of probe needle, which has a larger region than the conductive line 12. In the drawing, there is a ground plane 13 next to the routing layer 11 as an electrical reference.
In high-frequency circuit, the printed circuit board with the scale of mil should be designed to prevent the response degradation of multi-layer board. The degradation occurring is like the metal mounting pads due to a tiny distance between ground plane 13 and routing layer 11. The parasitic effect appearing between the layers will degrade the performance and frequency response of the PCB, especially in the region of surface-mounted device 16 as shown in FIG. 1.
FIG. 2 is the diagram of curves of the parasitic capacitor from the mounting pads in high-frequency circuit. The curve 21 and the curve 23 respectively show the capacitor effect while the distance between the routing layer 11 and the ground plane 13 is 4 mils and 12 mils. Thus, the capacitance shown on curve 21 is larger than the value shown on curve 23; therefore, the parasitic effect should be considered as the devices on PCB are thinner and smaller.
For improving the demerit of the prior art, in which a severe parasitic effect occurs between the layers in current PCBs because the width of a PCB is too narrow, the present invention provides a ground plane with at least one etching hole etched therein and corresponding to SMD on the routing layer.